Technical Field
[0001] The present invention relates to methods for inducing stress patterns in a polymeric
sheet to provide a desired contour for the sheet, and in two important aspects, to
providing such stress patterns in a sheet from which a shim in a cassette may be formed,
and the resultant structure of the shim.
Background Art
[0002] Audio cassettes of the dual spool type commonly include shims between side surfaces
of the spools and a case of the cassette. The purpose of the shims is to reduce friction
against the sides of the spools and coils of tape wound around the spools as they
rotate during use of the cassette, and to guide the tape as it is wound onto and off
of the spools. Typically the shims have a plurality of raised portions formed by creasing
or embossing that are found on various portions of the shim and may extend in various
directions such as longitudinally of the shim or radially of the spools, which raised
portions are intended to help these functions, and various patents such as U.S. Patent
Number 3,891,159 describe various patterns for these creases.
[0003] Such raised portions in a shim, however, do not always provide the degree of resilience
for the shim that is desirable to center the spools and coils of tape, and the raised
portions of the shims can provide areas of high wear concentration and can catch on
portions of the spools or wound tape as the spools are rotated to move the tape therebetween,
thus causing irregular movement of the tape and resultant distortions of the signals
being read from the tape.
Disclosure of Invention
[0004] The present invention provides a method for causing a polymeric sheet to be stressed
so that it normally has a predetermined uniform surface profile, which sheet can be
stressed so that it can be used in forming shims with a novel structure that provides
a high level of spool centering, tape guiding and friction reduction without resorting
to the types of raised portions found in the prior art types of shims described above.
[0005] According to the present invention there is provided a shim adapted to be inserted
between a case and side surfaces of spools in a cassette, which shim is of a resilient
elastic polymeric material, and comprises a first layer-like portion of the shim defining
a first surface of the shim adapted to contact the side surfaces of the spools, and
a second layer-like portion on the side of the first layer-like portion opposite the
first surface, with the first layer-like portion applying a generally uniform stress
in all directions along the second layer-like portion to bow the shim and cause the
first surface to be normally convex.
[0006] The term "layer-like portions" used herein with reference to the shim does not refer
to separately identifiable layers of the shim (although it could if the shim had a
laminate construction), but rather to portions of the shim that lie adjacent its opposite
major surfaces.
[0007] When two such bowed shims are deflected and positioned on opposite sides of the spools
with their convex first surfaces contacting the side surfaces of the spools, the shims
will resiliently bias the spools toward a central position therebetween and will serve
to improve guiding of the tape onto and off of the spools.
[0008] The bowed profile of the shims could be caused by several methods including drawing
a thin sheet or film of the polymeric material from which the shim is to be made over
an arcuate lip or domed surface to stretch and permanently deform one layer-like portion
of the film more than the opposite layer-like portion thereof, or conceivably even
by heat treatment of one of the film surfaces. The preferred method, however, comprises
causing particles or beads to impact at least one of the surfaces of the film until
such layer-like portions are formed.
[0009] Such impacting can be done in several ways, which may include moving a strip of the
polymeric material from which the shim is to be made past a source of the particles
being propelled via air pressure such as a dry blasting system in which small diameter
glass spheres (0.001-0.003 mm in diameter) are propelled through a nozzle.
[0010] Such impacting at one energy level will cause more expansion of the layer-like portion
of the material having the surface being impacted than the opposite layer-like portion
of the material so that the more expanded layer-like portion adjacent the impacted
surface applies a generally uniform stress in all directions along the opposite layer-like
portion. This will cause the material to bow so that the impacted surface will become
cylindrically convex along the strip, and will cause small portions, such as a shim,
cut from the strip of polymeric material to bow so that the impacted surface is spherically
convex.
[0011] Impacting the strip with particles at a slightly higher energy level will begin to
also cause expansion of the layer-like portion of the strip opposite the surface being
impacted so that the strip material will lose its bow and its surfaces will become
planar; and impacting the strip with particles at even higher energy levels will cause
the layer-like portion of the strip opposite the surface being impacted to be expanded
more than the layer-like portion having the impacted surface so that the material
bows and the impacted surface bacomes concave.
[0012] Examples of elastic resilient sheet materials that respond to the impacting by particles
as indicated above and are suitable for use in making bowed shims for cassettes include
thin sheets or films of ultra high molecular weight high density polyethylene which
is carbon or graphite filled to provide improved dielectric and lubricating properties;
and thin sheets or films of a polyester such as polyethylene terephthalate which is
coated with graphite for dielectric and lubricating properties. Shims made from films
of such materials have a sufficiently high modulus of elasticity to centrally bias
the spools and coils of tape on the spools of cassettes in which they are inserted.
[0013] In addition to being useful in the method for forming shims of the type described
above, such impacting of thin polymeric sheets or films provides a method for forming
a uniform planar, concave or convex surface on a thin polymeric sheet or film initially
having either an undesirable uniform surface contour or an undulating surface due
to stresses included in the film during its manufacture. Typically the method used
for this purpose comprises causing particles to impact one of the surfaces of the
film until a layer-like portion of the film adjacent one of its surfaces applies a
uniform stress in all directions along a layer-like portion adjacent its other surface
to cause the film to have a desired surface profile which can be convex or concave
as described above. The method can also include a step of subsequently impacting the
other surface of the film to provide the final shape for the surface of the film,
particularly where it is desired to minimize surface imperfections on both surfaces
of the film.
Brief Description Of Drawings
[0014] The present invention will be further described with reference to the accompanying
drawings wherein like numbers refer to like parts in the several views and wherein:
Figure 1 is a plan view of a cassette including a pair of shims according to the present
invention and having parts broken away to show details;
Figure 2 is a perspective view of one of the shims shown in Figure 1;
Figure 3 is a sectional view of the shim shown in Figure 2;
Figure 4 is a schematic view of a method according to the present invention for forming
the shim shown in Figures 1, 2 and 3; and
Figures 5, 6, and 7 schematically show the effect of impacting thin polymeric sheet
material with particles at different levels of energy in accordance with the method
illustrated in Figure 3.
Detailed Description
[0015] Referring now to the drawings, there is shown in Figure 1 a cassette 10 including
a pair of shims according to the present invention generally designated by the reference
numeral 12.
[0016] The cassette 10 is a dual spool cassette which includes a two-part case 14; two spools
16 rotatably mounted in aligned axially parallel spaced relationship within the case
14; a length of magnetizable tape 18 having opposite end portions helically wound
into coils about the spools 16; means including a pair of rollers 20 for defining
a path for a portion of the tape 18 extending between the spools 16; and the two shims
12 on the opposite sides of the spools 16 between the spools and side walls of the
case 14 which bias the spools 16 and coils of tape to a position centrally between
the sidewalls of the case, guide the tape 18 moving onto and off of the coils of tape
about the spools 16, and provide a low friction surface against which the spools 16
and tape 18 may move.
[0017] The shim 12 according to the present invention, best seen in Figures 2 and 3, is
a sheet of thin (e.g. 0.1 mm thick) resilient elastic polymeric material which is
generally rectangular in shape except for rounded corners, has opposite locating notches
22 centered along its edges adapted to receive locating posts 24 within the case 14,
has a central rectangular opening 26 to allow viewing the size of the coils of tape
on the spools 16 through the transparent case 14, and has spaced circular openings
28 which are positioned around access openings 30 for the spools 16 in the case 14.
The shim 12 comprises a first layer-like portion adjacent the spools 16 and a second
layer-like portion adjacent the case 14 with the first layer-like portion applying
a stress in all directions along the second layer-like portion to bow the shim 12
and form a normally convex outer surface 36 on its first layer-like portion. When
the shims 12 are in the cassette 10, the shims 12 are deflected and the convex surfaces
36 of the shims 12 contact with opposite side surfaces of the spools 16 and coils
of the tape 18 around the spools 16 so that the shims 12 resiliently bias the spools
16 and coils of tape to a central position in the case 14.
[0018] The term "layer-like portions" used herein with reference to the shim 12 does not
refer to separately identifiable layers of the shim 12 (although it could if the shim
had a laminate construction), but rather to portions of the shim that lie adjacent
its opposite major surfaces.
[0019] The method for making the shim 12 is best seen in Figure 4. As is schematically illustrated
therein, the method generally includes (1) providing a thin sheet or film of resilient
elastic polymeric material, which as illustrated may be a strip 40 of the material
extending between a supply roll 42 and a take-up roll 44 and may be conveyed therebetween
by rotation of the rolls 42, 44; (2) causing particles to impact at least one surface
46 of the material 40 as the material passes over a platen 47 until a layer-like portion
of the material 40 adjacent one of its surfaces applies a stress in all directions
along a layer-like portion of the material adjacent the other of its surfaces to cause
the material 40 to bow uniformly (which as illustrated may be done via air pressure
through a nozzle 48 via well known particle blasting techniques within an enclosure
50); and (3) cutting the shim 12 from the sheet of material, which as illustrated
may be done with a die 54.
[0020] As an example, shims 12 of 0.1 mm thick carbon filled ultra high molecular weight
high density polythylene which are impacted by particles so that, as a test procedure,
they provide a maximum bow of about 6.4 millimeters after they have been annealed
in a 76.5°C oven for 10 minutes (which annealing approximates the annealing that will
naturally occur during the service life of the shim) have been found very satisfactory
for use in dual spool cassettes when deflected to about 3.0 mm from a flat plane upon
assembly.
[0021] As is schematically illustrated in Figures 5, 6 and 7, impacting the surface 46 of
the strip of material 40 with particles at a first level of energy will cause the
layer-like portion of the material 40 adjacent the impacted surface 46 to expand more
than the layer-like portion adjacent an opposite surface 56 of the material 40, and
will thus cause the material 40 to bow so that the impacted surface 46 becomes convex
and the opposite surface becomes concave (Figure 5); impacting the surface 46 with
particles at a somewhat higher level of energy will cause the layer-like portion adjacent
the opposite surface 56 to expand about the same as the layer-like portion adjacent
the impacted surface 46 so that the material 40 is not bowed and both major surfaces
46 and 56 of the material 40 will be planar (Figure 6, long lengths of uniformly planar
material would be difficult to form by this method, however); and impacting the surface
46 with particles at yet a higher level of energy will cause the layer-like portion
adjacent the opposite surface 56 to expand more than the layer-like portion adjacent
the impacted surface 46 so that the material 40 will bow and the impacted surface
46 will become concave and the opposite surface 56 will become convex (Figure 7).
[0022] The method can thus be used to form a desired shape for the material with a convex,
planar or concave surface on either the impacted or nonimpacted surface. It is possible
to retain an original finish on one surface of a material and still shape that surface
as desired by impacting the opposite surface of the material, which may be desirable
in some instances. Alternatively, it is possible to reduce irregularities in the original
finish on one surface of a material and simultaneously shape that surface by impacting
it with the particles. The impacted surface of some materials appears to have lower
coefficients of friction than their original finishes, which may be caused by reducing
the size of irregularities. It is also possible to shape the surfaces of the material
by impacting both surfaces of the material either simultaneously or sequentially,
which may also be desirable in some instances.
[0023] A particularly useful result of the method is that it will remove undesirable uniform
or nonuniform stresses induced in the material during its manufacture or otherwise
while a desired uniform stress pattern is induced in the material to provide a desired
material and surface shape (e.g. convex, planar, or concave). The particle impacting
causes stress in the material that makes it bow so that its surfaces are normally
generally spherically convex or spherically concave. When the material is impacted
in a strip as illustrated in Figure 4, however, the strip will attain only cylindrically
concave and convex surfaces along a longitudinal axis until sections are severed from
the strip, whereupon the spherically concave and convex nature of the surfaces will
be apparent.
1. A cassette (10) including a case (14), two spools (16) rotatably mounted in aligned,
axially parallel, spaced relationship within the case (14), a length of tape (18)
having opposite end portions helically wound in coils about the spools (16) and having
a portion extending therebetween, means on the case defining a path for the portion
of tape (18) extending between the coils (16), and a pair of shims (12) of resilient
elastic polymeric material on the opposite sides of said spools (16) between said
spools (16) and said case (14), characterized in that said shims (12) each comprise
a first layer-like portion adjacent said spools (16) and a second layer-like portion
adjacent said case (14) with said first layer-like portion applying a stress in all
directions along said second layer-like portion to form a normally convex outer surface
(36) on said first layer-like portion, said shim (12) being resiliently deflected
between said case (14) and spools (16) to bias said spools (16) to central positions
therebetween.
2. A cassette (10) according to claim 1 further characterized in that said polymeric
material is carbon filled ultra high molecular weight high density polyethylene.
3. A cassette (10) according to claim 1 further characterized in that said polymeric
material is graphite coated polyester.
4. A method for forming a shim (12) adapted to be inserted between a case (14) and
side surfaces of spools (16) in a cassette (10), said method characterized by the
steps of:
providing a sheet (40) of resilient elastic polymeric material having opposite surfaces
(46, 56);
impacting at least one of the surfaces (46) of the sheet (40) with particles until
a layer-like portion of the sheet (40) adjacent one of the surfaces (46) applies a
stress in all directions along a layer-like portion of the sheet (40) adjacent the
other of the surfaces (56) to cause the sheet (40) to bow uniformly; and
cutting the sheet (40) to form the shim (12).
5. A method according to claim 7, further characterized by impacting said one surface
(46) of the sheet (40) with particles until the impacted surface (46) is convex.
6. A method according to claim 7 further characterized by impacting said one surface
(46) of the sheet (40) with particles until the impacted surface (46) is concave.